Preliminary health-impact assessment results

Introduction

This document describes and presents initial results for the health-impact assessment related to adding more native plants to City of Denver. We aim to estimate the number of deaths that would be prevented by adding more green space, specifically native plants, to the City of Denver. We estimate the number of deaths prevented corresponding to three broad greening scenarios, which were informed by conversations with local stakeholders. The scenarios are:

• Add native plants to all census block groups such that
  • 100% of the block group’s area is as green as native plants
  • 20% of the block group’s area is as green as native plants
• Create native buffers around riparian areas (rivers, streams, lakes, and ponds) of the following sizes:
  • 200 feet (ideal for ecosystem health but possibly not realistic)
  • 100 feet (compromise)
  • 50 feet (most realistic; less good for ecosystem health)
• Replace a portion of parking-lot surface with native plants:
  • 100% of the existing parking area
  • 50% of the existing parking area
  • 20% of the existing parking area

For all scenarios, we measured green-space exposure by the normalized difference vegetation index (NDVI) measured by the Landsat-8 satellite at a spatial resolution of 30 meters squared. To determine the target NDVI value, we measured the NDVI of a 100% native zone in the Denver Botanic Gardens on several cloud-free spring and summer days. The NDVI value was about 0.5 in this area, so we assumed that, by adding more native plants to places in Denver, the NDVI of those areas would change to 0.5.

Scenario 1: add native plants to all census block groups

We first measured the mean NDVI of each census block group on July 4, 2021. The weighted block-group-level mean is presented here, weighted by the proportion each 30 square-meter pixel covers by the census block group. For example, if half of a pixel overlaps the block group, it receives a weight of 0.5 in the weighted average. We also removed bodies of water before measuring NDVI.

We excluded census block whose baseline NDVI was above the native-plant threshold:

Scenario 2: add native-plants buffers to riparian areas

We measured NDVI in a 200-foot buffer, a 100-foot buffer, and a 50-foot buffer around all bodies of water in Denver. We downloaded bodies of water from OpenStreetMap (code here. We defined residential exposure to these riparian areas as those individuals living within a 500-meter buffer, following the green-space literature that has defined green-space exposure based on residential proximity. We estimated the number of people in this buffer by multiplying the population density of the census block group by the intersecting area.

The below map depicts mean NDVI in the portions of census block groups that intersect a 200-foot buffer as well as those pieces that intersect the part of the 500 m buffer that would not be intervened upon, i.e., the part between the 200-foot buffer and the edge of the 500 m buffer.

Scenario 4: add native plants to parking lots

We obtained spatial data on parking lots from the City of Denver Open Data Catalog. About 9% of the city is covered with parking lots:

## # A tibble: 1 × 4
##     area_ft2 area_mi2 den_area_mi2_total percent_area_covered_by_parking
##        <dbl>    <dbl>              <dbl>                           <dbl>
## 1 376594381.     13.5                155                          0.0872

We similarly measured the NDVI on the parking lots as well as the NDVI in the census block groups that are within a 500-m buffer radius of any parking lots (which is most).

We present a small subset, as the file size with the full map rendered exceeds GitHub’s file-upload limits:

Results

We estimated the number of deaths averted under each scenario by following a recent meta-analyses that estimated that for every 0.1 unit increase in exposure to NDVI, the relative risk of death decreases about 4% (pooled risk ratio of 0.96, 95% confidence interval (CI): 0.94, 0.97)1. We defined NDVI exposure as that within a 500-meter buffer of the individual’s place of residence. We used this risk ratio to estimate the population attributable fraction corresponding to the proposed change in NDVI from the baseline level to the native-plants level for each census block group (first scenario) or the part of the census block group intersecting the proposed intervention area (riparian and parking scenarios). We then estimated the number of deaths prevented considering estimates of population and all-cause mortality rates. We used 5-year American Community Survey Data (2015-2019) to estimate the population in 5-year age groups in each census block group and age-stratified estimates of mortality rates for the state of Colorado from the Global Burden of Disease project. If an intervention area did not cover an entire census block group, we multiplied the population density of the block group in that age group by the area of the block group covered by the intervention area to estimate the total population in the area affected by the intervention. We restricted analyses to adults aged 30 and above following the age range of many of the cohort studies reviewed. We also restricted analyses to those census block groups or census-block-group pieces below the native-plants NDVI threshold.

scenario	scenario_sub	pop_affected	attrib_deaths
all-bg	20-pct	385372.0	-45.778418
all-bg	100-pct	385372.0	-236.401033
riparian	200-ft	176676.0	-7.564989
riparian	100-ft	176676.0	-2.477237
riparian	50-ft	176676.0	-1.072617
prkng	100-pct-prkng	377598.6	-40.364892
prkng	50-pct-prkng	377598.6	-20.082410
prkng	20-pct-prkng	377598.6	-8.009128